Page 1 Overview and Issues of the MEIC Interaction Region M. Sullivan MEIC Accelerator Design Review September 15-16, 2010

Page 2 Outline MEIC IR and detector Backgrounds SR Radiative Bhabhas Summary Conclusion

Page 3 MEIC Interaction Region Design The MEIC Interaction Region Features 50 mrad crossing angle Detector is aligned along the electron beam line Electron FF magnets start/stop 3.5 m from the IP Proton/ion FF magnets start/stop 7 m from the IP

Page 4 Table of Parameters (electrons) Electron beam Energy range 3-11 GeV Beam-stay-clear 12 beam sigmas Emittance (  x /  y ) (5 GeV) (5.5/1.1) nm-rad Betas  x * = 10 cm  x max = 435 m  y * = 2 cm  y max = 640 m Final focus magnets NameZ of face L (m) k G (11 GeV) QFF QFF QFFL

Page 5 Table of Parameters (proton/ion) Proton/ion beam Energy range GeV Beam-stay-clear 12 beam sigmas Emittance (  x /  y ) (60 GeV) (5.5/1.1) nm-rad Betas  x * = 10 cm  x max = 2195 m  y * = 2 cm  y max = 2580 m Final focus magnets NameZ of face L (m) k G (60 GeV) QFF QFF QFFL

Page 6 Magnet apertures The magnet apertures are usually set by the required BSC Making the BSC generous forces larger aperture final focus magnets – usually these are more difficult magnets to build However, a large BSC improves machine flexibility by allowing for smaller beta* values (and hence larger beta max values) if the accelerator can operate with fewer beam sigmas than that defined by the BSC and backgrounds are acceptable The MEIC 12 sigma BSC definition is a reasonable compromise

Page 7 Interaction Region and Detector EM Calorimeter Hadron Calorimeter Muon Detector EM Calorimeter Solenoid yoke + Hadronic Calorimeter Solenoid yoke + Muon Detector HTCC RICH Tracking 5 m solenoid IP Ultra forward hadron detection dipole Low-Q 2 electron detection Large aperture electron quads Small diameter electron quads ion quads Small angle hadron detection dipole Central detector with endcaps ~50 mrad crossing Courtesy Pawel Nadel-Tournski and Alex Bogacz

Page 8 Estimate of the detector magnetic field (B z ) QFF1 QFF2 QFFL QFFP ~2 kG Tesla The detector magnetic field will have a significant impact on the beams. Some of the final focusing elements will have to work in this field.

Page 9 Energy range Both beam energies have a fairly large energy range requirement The final focus elements must be able to accommodate these energy ranges An attractive alternative for some of the final focusing elements (especially the electron elements) is to use permanent magnets – they have a very small size and do not need power leads However, any PM design has to be able to span the energy range (for instance, only 30% of the final focus strength can be PM)

Page 10 First look at SR backgrounds x x W 38 2 Synchrotron radiation photons incident on various surfaces from the last 4 electron quads Rate per bunch incident on the surface > 10 keV Rate per bunch incident on the detector beam pipe assuming 1% reflection coefficient and solid angle acceptance of 4.4 % M. Sullivan July 20, 2010 F$JLAB_E_3_5M_1A 50 mrad Beam current = 2.32 A 2.9x10 10 particles/bunch X P+P+ e-e- Z Electron energy = 11 GeV  x /  y = 1.0/0.2 nm-rad

Page 11 5 times larger beam emittances and lower beam energy 1.8x x x x W 7 4 Synchrotron radiation photons incident on various surfaces from the last 4 electron quads Rate per bunch incident on the surface > 10 keV Rate per bunch incident on the detector beam pipe assuming 1% reflection coefficient and solid angle acceptance of 4.4 % M. Sullivan July 20, 2010 F$JLAB_E_3_5M_1A 50 mrad Beam current = 2.32 A 2.9x10 10 particles/bunch X P+P+ e-e- Z Electron energy = 5 GeV  x /  y = 5.5/1.1 nm-rad

Page 12 Beam tails Assumed beam tail distribution is the same as was used in PEP-II background calculations The tail distribution is primarily driven by the beam lifetime The SR background is dominated by the beam tail particles

Page 13 Backgrounds Initial look at synchrotron radiation indicates that this background should not be a problem but a more thorough study is needed Need to look at lost particle backgrounds for both beams Proton beam has been studied extensively Generally one can restrict the study to the region upstream of the IP before the last bend magnet A high quality vacuum in this region is sometimes enough

Page 14 Radiative Bhabha Background There is a luminosity background from the electron beam During the collision the electron can radiate a photon This was a major source of neutrons in the B-Factory detectors

Page 15 B-Factory Radiative bhabhas Off energy beam particles were swept out of the beam and then hit the local vacuum pipe

Page 16 B-Factory Radiative Bhabhas Both B-factories experienced this luminosity background

Page 17 Super B-factories The super B-factories (SuperB and superKEKB) have designed the IR so that this background source is minimized This background has become one of the primary design drivers for the interaction region The other background that is close to being a design driver is the two-photon process where the produced background comes from low-energy e+/e- pairs produced from the interaction This background should not be a factor in the MEIC design (down by another factor of  )

Page 18 MEIC The radiative bhabha background may not be as important for the MEIC as it was for the B-factories but there will be beam induced backgrounds in the low angle detectors for the low-Q 2 detectors and this reaction will be very similar to the physics signal looked for here IP Ultra forward hadron detection dipole Low-Q 2 electron detection Large aperture electron quads Small diameter electron quads ion quads Small angle hadron detection dipole Central detector with endcaps ~50 mrad crossing

Page 19 Summary The IR is one of the more challenging regions to design There are multiple constraints, however, balancing the various requirements to maximize the physics is always the primary goal of any design The beam induced backgrounds many times control a large part of the design of the interaction region in order to allow the detector to operate in what can be a very hostile environment

Page 20 Conclusion A careful study of the beam induced backgrounds is always an important aspect of any interaction region design As the accelerator design evolves one must constantly recheck backgrounds to make sure the changes do not have an adverse effect on rates There is always room for unexpected backgrounds so diligence in controlling and understanding backgrounds always tends to pay off